The present invention relates to a method and apparatus for detecting pattern defects in products such as printed writing boards, IC mask patterns, lead frames and the like.
In pattern defect detection, there are many kinds of defects to detect as, for example, a mouse bite, a bump, a disconnection, a widening, a narrowing, a bridging, a pin holing and remaining copper. These defects are illustrated in FIG. 15 by a mouse bite portion 2, a bump portion 4, a copper disconnection 6, a wide copper portion 8, a narrow copper portion 10, a bridge portion 12, a pin hole portion 14 and a remaining copper portion 16. Detecting such pattern defects on printed wiring boards and the like is mainly carried out in the prior art by a pattern matching method and by a design rule check method.
In the conventional "pattern matching" method, an image pattern of a reference object is overlapped and compared to an image pattern of an object to be inspected, and then the pattern defects are evaluated by detecting discrepancies in image patterns between the reference object and the object being inspected. This method is described, for example, in Japanese Patent Kokoku Publication No. 2069/1984, Japanese Patent Kokai Publication No. 61604/1985 and Japanese Patent Kokai Publication No. 140009/1987.
In the known "design rule check" method, various features of a reference image pattern, such as line width, angle, specific pattern, defective form and the like, are stored and compared with the features of an image pattern to be inspected, and then the pattern defects are evaluated by detecting image patterns which are different from all of the reference features, or by noting forms which are known to be defective. This method is, for example, known from Japanese Patent Kokai Publication No. 167649/1982 and Japanese Patent Kokai Publication No. 149905/1982.
The aforementioned methods have respective merits and disadvantages in their pattern defects detecting ability. For example, the pattern matching method can detect the pattern defects without limitation as to form. In a case where image patterns are read in high resolution, however, it is possible to detect image discrepancies which are not real pattern defects but which are caused by quantization errors, arising from quantization procedures and forming errors, others which arise during forming of image patterns on the objects, which cause harmless deviations, between the reference object and the objects being fabricated.
In the design rule check method, it is possible to detect the pattern defects without the reference image pattern which is essential to the pattern matching method, and it has superior ability for detecting fine pattern defects. However, in a case of inspecting a character, a special figure as shown in FIG. 16 and a power supply line on a multi-layer print wired board applicable to high density wiring, it is likely for a scrap portion 18, projections portions 20 and 22, a disconnection portion 24 and an undersized portion 26 to be adjudged as a pattern defect by the error recognition procedure even though they are not real pattern defects.
Although detection of the pattern defects was carried out by either the pattern matching method and the design rule check method, the detecting method was not always applicable to the image patterns in the prior art since each method had its aforementioned peculiar disadvantages. Therefore, productivity and yield in the prior art methods for testing suffered, in part due to its consideration of information which was not relevant to the pattern inspection of the object.